TRP Channel Knockout Mice Lose Their Cool

Divergent sensory pathways of sneezing and coughing

Sneezing and coughing are primary symptoms of many respiratory viral infections and allergies. It is generally assumed that sneezing and coughing involve common sensory receptors and molecular neurotransmission mechanisms. Here, we show that the nasal mucosa is innervated by several discrete populations of sensory neurons, but only one population (MrgprC11+MrgprA3-) mediates sneezing responses to a multitude of nasal irritants, allergens, and viruses. Although this population also innervates the trachea, it does not mediate coughing, as revealed by our newly established cough model. Instead, a distinct sensory population (somatostatin [SST+]) mediates coughing but not sneezing, unraveling an unforeseen sensory difference between sneezing and coughing. At the circuit level, sneeze and cough signals are transmitted and modulated by divergent neuropathways. Together, our study reveals the difference in sensory receptors and neurotransmission/modulation mechanisms between sneezing and coughing, offering neuronal drug targets for symptom management in respiratory viral infections and allergies.

Hypothalamic Neuromodulation of Hypothermia in Domestic Animals

When an organism detects decreases in their core body temperature, the hypothalamus, the main thermoregulatory center, triggers compensatory responses. These responses include vasomotor changes to prevent heat loss and physiological mechanisms (e.g., shivering and non-shivering thermogenesis) for heat production. Both types of changes require the participation of peripheral thermoreceptors, afferent signaling to the spinal cord and hypothalamus, and efferent pathways to motor and/or sympathetic neurons. The present review aims to analyze the scientific evidence of the hypothalamic control of hypothermia and the central and peripheral changes that are triggered in domestic animals.

Assessing the Effects of the Topical Application of L-Menthol on Pain-Related Somatosensory-Evoked Potentials Using Intra-Epidermal Stimulation

L-menthol is known to activate transient receptor potential melastatin 8 (TRPM8) and induce analgesia to thermal stimuli. However, since thermal stimulation leads to the interaction among the other TRP channels, it was unclear whether L-menthol causes analgesia to stimuli other than thermal stimuli. Therefore, we aimed to investigate whether activating TRPM8 via topical application of 10% menthol solution attenuates pain-related somatosensory-evoked potentials (pSEPs) and affects numerical rating scale (NRS) score using intra-epidermal electrical stimulation (IES). We applied 10% L-menthol or control solution on the dorsum of the right hand of 25 healthy participants. The pSEP and NRS, elicited by IES, and sensory threshold were measured before and after each solution was applied. The results showed that the topical application of 10% L-menthol solution significantly reduced N2-P2 amplitude in pSEPs compared with the control solution. Moreover, the N2 latency was significantly prolonged upon the topical application of L-menthol solution. NRS scores were similar under both conditions. These results suggest that topical application of L-menthol does not alter subjective sensation induced using IES, although it may attenuate afferent signals at free nerve endings even with stimuli that do not directly activate TRP channels.

The distinctive role of menthol in pain and analgesia: Mechanisms, practices, and advances

Menthol is an important flavoring additive that triggers a cooling sensation. Under physiological condition, low to moderate concentrations of menthol activate transient receptor potential cation channel subfamily M member 8 (TRPM8) in the primary nociceptors, such as dorsal root ganglion (DRG) and trigeminal ganglion, generating a cooling sensation, whereas menthol at higher concentration could induce cold allodynia, and cold hyperalgesia mediated by TRPM8 sensitization. In addition, the paradoxical irritating properties of high concentrations of menthol is associated with its activation of transient receptor potential cation channel subfamily A member 1 (TRPA1). Under pathological situation, menthol activates TRPM8 to attenuate mechanical allodynia and thermal hyperalgesia following nerve injury or chemical stimuli. Recent reports have recapitulated the requirement of central group II/III metabotropic glutamate receptors (mGluR) with endogenous κ-opioid signaling pathways for menthol analgesia. Additionally, blockage of sodium channels and calcium influx is a determinant step after menthol exposure, suggesting the possibility of menthol for pain management. In this review, we will also discuss and summarize the advances in menthol-related drugs for pathological pain treatment in clinical trials, especially in neuropathic pain, musculoskeletal pain, cancer pain and postoperative pain, with the aim to find the promising therapeutic candidates for the resolution of pain to better manage patients with pain in clinics.

The Influence of Dietary Supplementations on Neuropathic Pain

Neuropathic pain is defined as pain caused by a lesion or disease of the somatosensory nervous system and affects 7-10% of the worldwide population. Neuropathic pain can be induced by the use of drugs, including taxanes, thus triggering chemotherapy-induced neuropathic pain or as consequence of metabolic disorders such as diabetes. Neuropathic pain is most often a chronic condition, and can be associated with anxiety and depression; thus, it negatively impacts quality of life. Several pharmacologic approaches exist; however, they can lead numerous adverse effects. From this perspective, the use of nutraceuticals and diet supplements can be helpful in relieve neuropathic pain and related symptoms. In this review, we discuss how diet can radically affect peripheral neuropathy, and we focus on the potential approaches to ameliorate this condition, such as the use of numerous nutritional supplements or probiotics.

Transient Receptor Potential Channels and Botulinum Neurotoxins in Chronic Pain

Pain afflicts more than 1.5 billion people worldwide, with hundreds of millions suffering from unrelieved chronic pain. Despite widespread recognition of the importance of developing better interventions for the relief of chronic pain, little is known about the mechanisms underlying this condition. However, transient receptor potential (TRP) ion channels in nociceptors have been shown to be essential players in the generation and progression of pain and have attracted the attention of several pharmaceutical companies as therapeutic targets. Unfortunately, TRP channel inhibitors have failed in clinical trials, at least in part due to their thermoregulatory function. Botulinum neurotoxins (BoNTs) have emerged as novel and safe pain therapeutics because of their regulation of exocytosis and pro-nociceptive neurotransmitters. However, it is becoming evident that BoNTs also regulate the expression and function of TRP channels, which may explain their analgesic effects. Here, we summarize the roles of TRP channels in pain, with a particular focus on TRPV1 and TRPA1, their regulation by BoNTs, and briefly discuss the use of BoNTs for the treatment of chronic pain.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Resolvins: Potent Pain Inhibiting Lipid Mediators via Transient Receptor Potential Regulation

Chronic pain is a serious condition that occurs in the peripheral nervous system (PNS) and the central nervous system (CNS). It is caused by inflammation or nerve damage that induces the release of inflammatory mediators from immune cells and/or protein kinase activation in neuronal cells. Both nervous systems are closely linked; therefore, inflammation or nerve damage in the PNS can affect the CNS (central sensitization). In this process, nociceptive transient receptor potential (TRP) channel activation and expression are increased. As a result, nociceptive neurons are activated, and pain signals to the brain are amplified and prolonged. In other words, suppressing the onset of pain signals in the PNS can suppress pain signals to the CNS. Resolvins, endogenous lipid mediators generated during the resolution phase of acute inflammation, inhibit nociceptive TRP ion channels and alleviate chronic pain. This paper summarizes the effect of resolvins in chronic pain control and discusses future scientific perspectives. Further study on the effect of resolvins on neuropathic pain will expand the scope of pain research.

Cutaneous Vasodilation during Local Heating: Role of Local Cutaneous Thermosensation

We tested the hypothesis that cutaneous vasodilation during local skin heating in humans could be manipulated based upon the ability to desensitize TRPV4 ion channels by applying the thermal stimuli in a series of pulses. Each subject was instrumented with intradermal microdialysis probes in the dorsal forearm skin and perfused with 0.9% saline at 1.5 μl/min with local skin temperature controlled with a Peltier unit (9 cm²) at 34°C. Local skin temperature was manipulated for 50 min in two classic ways: a step increase to 38°C (0.1°C/s, n = 10), and a step increase to 42°C (n = 10). To desensitize TRPV4 ion channels local skin temperature was manipulated in the following way: pulsed increase to 38°C (1 pulse per min, 30 s duration, 1.0°C/s, n = 10), and 4) pulsed increase to 42°C (1.0°C/s, n = 9). Skin blood flow (SkBF, laser Doppler) was recorded directly over the middle microdialysis probe and the dialysate from all three probes were collected during baseline (34°C) and each skin heating period. The overall cutaneous vascular conductance (CVC) response to local heating was estimated from the area under the % CVCmax-time curve. The appearance of the neuropeptide calcitonin gene related peptide (CGRP) in dialysate did not change with skin heating in any protocol. For the skin temperature challenge of 34 to 38°C, the area under the % CVCmax-time curve averaged 1196 ± 295 (SD) % CVCmax•min, which was larger than the 656 ± 282% CVCmax•min during pulsed heating (p < 0.05). For the skin temperature challenge of 34 to 42°C, the area under the % CVCmax-time curve averaged 2678 ± 458% CVCmax•min, which was larger than the 1954 ± 533% CVCmax•min during pulsed heating (p < 0.05). The area under the % CVCmax•min curve, was directly proportional to the accumulated local skin thermal stress (in °C•min) (r² = 0.62, p < 0.05, n = 39). This association indicates a critical role of local integration of thermosensitive receptors in mediating the cutaneous vasodilator response to local skin heating. Given that we saw no differences in the levels of CGRP in dialysate, the role of the vasoactive peptide CGRP in the cutaneous vasodilator response to local skin heating is not supported by our data.

TRP Channels in Skin Biology and Pathophysiology

Ion channels of the Transient Receptor Potential (TRP) family mediate the influx of monovalent and/or divalent cations into cells in response to a host of chemical or physical stimuli. In the skin, TRP channels are expressed in many cell types, including keratinocytes, sensory neurons, melanocytes, and immune/inflammatory cells. Within these diverse cell types, TRP channels participate in physiological processes ranging from sensation to skin homeostasis. In addition, there is a growing body of evidence implicating abnormal TRP channel function, as a product of excessive or deficient channel activity, in pathological skin conditions such as chronic pain and itch, dermatitis, vitiligo, alopecia, wound healing, skin carcinogenesis, and skin barrier compromise. These diverse functions, coupled with the fact that many TRP channels possess pharmacologically accessible sites, make this family of proteins appealing therapeutic targets for skin disorders.

Tetrahydroisoquinoline-Derived Urea and 2,5-Diketopiperazine Derivatives as Selective Antagonists of the Transient Receptor Potential Melastatin 8 (TRPM8) Channel Receptor and Antiprostate Cancer Agents

Tetrahydroisoquinoline derivatives containing embedded urea functions were identified as selective TRPM8 channel receptor antagonists. Structure-activity relationships were investigated, with the following conclusions: (a) The urea function and the tetrahydroisoquinoline system are necessary for activity. (b) Bis(1-aryl-6,7dimethoxy-1,2,3,4-tetrahydroisoquinolyl)ureas are more active than compounds containing one tetrahydroisoquinoline ring and than an open phenetylamine ureide. (c) Trans compounds are more active than their cis isomers. (d) Aryl substituents are better than alkyls at the isoquinoline C-1 position. (e) Electron-withdrawing substituents lead to higher activities. The most potent compound is the 4-F derivative, with IC50 in the 10(-8) M range and selectivities around 1000:1 for most other TRP receptors. Selected compounds were found to be active in reducing the growth of LNCaP prostate cancer cells. TRPM8 inhibition reduces proliferation in the tumor cells tested but not in nontumor prostate cells, suggesting that the activity against prostate cancer is linked to TRPM8 inhibition.

Meningeal transient receptor potential channel M8 activation causes cutaneous facial and hindpaw allodynia in a preclinical rodent model of headache

BACKGROUND

Migraine headache is a neurological disorder affecting millions worldwide. However, little is known about the mechanisms contributing to migraine. Recent genome-wide association studies have found single nucleotide polymorphisms in the gene encoding transient receptor potential channel M8. Transient receptor potential channel M8 is generally known as a cold receptor but it has been implicated in pain signaling and may play a role in migraine pain.

METHODS

In order to investigate whether transient receptor potential channel M8 may contribute to the pain of migraine, the transient receptor potential channel M8 activator icilin was applied to the dura mater using a rat behavioral model of headache. Cutaneous allodynia was measured for 5 hours using Von Frey filaments.

RESULTS

Dural application of icilin produced cutaneous facial and hind paw allodynia that was attenuated by systemic pretreatment with the transient receptor potential channel M8-selective antagonist AMG1161 (10 mg/kg p.o.). Further, the anti-migraine agent sumatriptan (0.6 mg/kg s.c.) or the non-selective NOS inhibitor L-NAME (20 mg/kg i.p.) also attenuated allodynia when given as a pretreatment.

CONCLUSIONS

These data indicate that transient receptor potential channel M8 activation in the meninges produces behaviors in rats that are consistent with migraine and that are sensitive to pharmacological mechanisms known to have efficacy for migraine in humans. The findings suggest that activation of meningeal transient receptor potential channel M8 may contribute to the pain of migraine.

Meningeal afferent signaling and the pathophysiology of migraine

Migraine is the most common neurological disorder. Attacks are complex and consist of multiple phases but are most commonly characterized by intense, unilateral, throbbing headache. The pathophysiology contributing to migraine is poorly understood and the disorder is not well managed with currently available therapeutics, often rendering patients disabled during attacks. The mechanisms most likely to contribute to the pain phase of migraine require activation of trigeminal afferent signaling from the cranial meninges and subsequent relay of nociceptive information into the central nervous system in a region of the dorsal brainstem known as the trigeminal nucleus caudalis. Events leading to activation of meningeal afferents are unclear, but nerve endings within this tissue are mechanosensitive and also express a variety of ion channels including acid-sensing ion channels and transient receptor-potential channels. These properties may provide clues into the pathophysiology of migraine by suggesting that decreased extracellular pH and environmental irritant exposure in the meninges contributes to headache. Neuroplasticity is also likely to play a role in migraine given that attacks are triggered by routine events that are typically nonnoxious in healthy patients and clear evidence of sensitization occurs during an attack. Where and how plasticity develops is also not clear but may include events directly on the afferents and/or within the TNC. Among the mediators potentially contributing to plasticity, calcitonin gene-related peptide has received the most attention within the migraine field but other mechanisms may also contribute. Ultimately, greater understanding of the molecules and mechanisms contributing to migraine will undoubtedly lead to better therapeutics and relief for the large number of patients across the globe who suffer from this highly disabling neurological disorder.

Inhibition of TRPM8 channels reduces pain in the cold pressor test in humans

The transient receptor potential (subfamily M, member 8; TRPM8) is a nonselective cation channel localized in primary sensory neurons, and is a candidate for cold thermosensing, mediation of cold pain, and bladder overactivity. Studies with TRPM8 knockout mice and selective TRPM8 channel blockers demonstrate a lack of cold sensitivity and reduced cold pain in various rodent models. Furthermore, TRPM8 blockers significantly lower body temperature. We have identified a moderately potent (IC50 = 103 nM), selective TRPM8 antagonist, PF-05105679 [(R)-3-[(1-(4-fluorophenyl)ethyl)(quinolin-3-ylcarbonyl)amino]methylbenzoic acid]. It demonstrated activity in vivo in the guinea pig bladder ice water and menthol challenge tests with an IC50 of 200 nM and reduced core body temperature in the rat (at concentrations >1219 nM). PF-05105679 was suitable for acute administration to humans and was evaluated for effects on core body temperature and experimentally induced cold pain, using the cold pressor test. Unbound plasma concentrations greater than the IC50 were achieved with 600- and 900-mg doses. The compound displayed a significant inhibition of pain in the cold pressor test, with efficacy equivalent to oxycodone (20 mg) at 1.5 hours postdose. No effect on core body temperature was observed. An unexpected adverse event (hot feeling) was reported, predominantly periorally, in 23 and 36% of volunteers (600- and 900-mg dose, respectively), which in two volunteers was nontolerable. In conclusion, this study supports a role for TRPM8 in acute cold pain signaling at doses that do not cause hypothermia.

Artemin, a glial cell line-derived neurotrophic factor family member, induces TRPM8-dependent cold pain

Chronic pain associated with injury or disease can result from dysfunction of sensory afferents whereby the threshold for activation of pain-sensing neurons (nociceptors) is lowered. Neurotrophic factors control nociceptor development and survival, but also induce sensitization through activation of their cognate receptors, attributable, in part, to the modulation of ion channel function. Thermal pain is mediated by channels of the transient receptor potential (TRP) family, including the cold and menthol receptor TRPM8. Although it has been shown that TRPM8 is involved in cold hypersensitivity, the molecular mechanisms underlying this pain modality are unknown. Using microarray analyses to identify mouse genes enriched in TRPM8 neurons, we found that the glial cell line-derived neurotrophic factor (GDNF) family receptor GFRα3 is expressed in a subpopulation of TRPM8 sensory neurons that have the neurochemical profile of cold nociceptors. Moreover, we found that artemin, the specific GFRα3 ligand that evokes heat hyperalgesia, robustly sensitized cold responses in a TRPM8-dependent manner in mice. In contrast, GFRα1 and GFRα2 are not coexpressed with TRPM8 and their respective ligands GDNF and neurturin did not induce cold pain, whereas they did evoke heat hyperalgesia. Nerve growth factor induced mild cold sensitization, consistent with TrkA expression in TRPM8 neurons. However, bradykinin failed to alter cold sensitivity even though its receptor expresses in a subset of TRPM8 neurons. These results show for the first time that only select neurotrophic factors induce cold sensitization through TRPM8 in vivo, unlike the broad range of proalgesic agents capable of promoting heat hyperalgesia.

The molecular and cellular basis of cold sensation

Of somatosensory modalities, cold is one of the more ambiguous percepts, evoking the pleasant sensation of cooling, the stinging bite of cold pain, and welcome relief from chronic pain. Moreover, unlike the precipitous thermal thresholds for heat activation of thermosensitive afferent neurons, thresholds for cold fibers are across a range of cool to cold temperatures that spans over 30 °C. Until recently, how cold produces this myriad of biological effects has been poorly studied, yet new advances in our understanding of cold mechanisms may portend a better understanding of sensory perception as well as provide novel therapeutic approaches. Chief among these was the identification of a number of ion channels that either serve as the initial detectors of cold as a stimulus in the peripheral nervous system, or are part of rather sophisticated differential expression patterns of channels that conduct electrical signals, thereby endowing select neurons with properties that are amenable to electrical signaling in the cold. This review highlights the current understanding of the channels involved in cold transduction as well as presents a hypothetical model to account for the broad range of cold thermal thresholds and distinct functions of cold fibers in perception, pain, and analgesia.

Differential inhibition of the TRPM8 ion channel by Gαq and Gα 11

Cold temperature is encoded by the cold-sensitive ion channel TRPM8 in somatosensory neurons. It has been unclear how TRPM8 is modulated so that it can mediate distinct type of cold signaling. We have recently reported that activated Gαq directly inhibits TRPM8 after activation of Gq-coupled receptors. Here, we further show that activation of the muscarinic receptor M1R, which is known to inhibit M currents through PLCβ-mediated hydrolysis of PtdIns(4,5)P 2, similarly inhibited TRPM8 potently, but inhibition was not prevented by the PLC inhibitor U73122. Interestingly, although Gαq and Gα 11 are indistinguishable in activating PLCβ and hydrolysing PtdIns(4,5)P 2, activated Gα 11 inhibited TRPM8 to a lesser extent than activated Gαq. The differential TRPM8 inhibition is determined by a specific residue E197 on Gα 11, because mutating this residue to the corresponding residue on Gαq restored TRPM8 inhibition to a similar degree as mediated by Gαq. These results reinforce the idea that activated Gαq directly inhibits TRPM8 independently from PtdIns(4,5)P 2 hydrolysis-mediated inhibition of TRPM8.

Activation of TRPM8 cold receptor triggers allodynia-like behavior in spinally injured rats

Aims

Pain in response to innocuous cold stimulation (cold allodynia) is a common symptom in patients with neuropathic pain. Cold allodynia is difficult to treat and its mechanisms are poorly understood. Several transient receptor potential (TRP) channels have been shown to be the molecular sensors for cold stimulation in a temperature-dependent manner, but the contribution of various TRP channels in mediating cold allodynia in neuropathic pain is unclear. We have previously shown that spinally injured rats developed neuropathic pain-like behaviors, including marked cold allodynia. We now assessed the role of TRP channels in mediating cold allodynia in rats after ischemic spinal cord injury.

Methods

Methods: Spinal cord injury was produced using a photochemical method. The mechanical allodynia was assessed by examining the vocalization thresholds to graded mechanical touch/pressure applied with von Frey hairs. Temperature controlled cold stimulation was produced by a Peltier thermode (active surface 25 mm × 50 mm) connected to a MSA Thermal Simulator (Somedic, Sweden) with baseline temperature of 32 °C. The rate of temperature change was 0.5 °C/s. The temperature required to elicit cold allodynia was examined. The responses of the rats to topical application of icilin or menthol, agonists of transient receptor potential melastain 8 (TRPM8), were also studied.

Results

Normal rats did not exhibit nociceptive responses to cooling stimulation to the trunk and back area (minimal temperature +6°C) and they also did not react aversively to topical application of icilin or menthol. After spinal cord injury, the rats developed mechanical allodynia at the trunk and back just rostral to the dermatome of the injured spinal segments. In the same area, rats exhibited significant nociceptive responses to cooling from day 1 after injury, lasting for at least 70 days which is the longest time of observation. For the first two weeks after injury, the majority of spinally injured rats had a nociceptive response to cooling above 17°C. At day 70, about 50% of rats responded to cooling above 17 °C. Topical application of 400 μM icilin or 4mM menthol also elicited pain-like responses in spinally injured rats and these two cold mimetics also significantly exacerbated existing mechanical allodynia.

Conclusion

Our results showed that activation of the TRPM8 channel by menthol or icilin triggers allodynia in spinally injured rats and increases, rather than decreases, mechanical allodynia. TRPM8 channels which respond to cooling above 17 ° C may be involved at least in part in mediating cold allodynia in the rat model of neuropathic spinal cord injury pain.

Implications

The work introduced a method of quantitative testings of responses of rats to cold stimulation and may contribute to the understanding of mechanisms of cold allodynia after injury to the nervous system.

Direct inhibition of the cold-activated TRPM8 ion channel by Gαq

Activation of the TRPM8 ion channel in sensory nerve endings produces a sensation of pleasant coolness. Here we show that inflammatory mediators such as bradykinin and histamine inhibit TRPM8 in intact sensory nerves, but do not do so via conventional signalling pathways. The G-protein subunit Ga_q instead binds to TRPM8 and when activated by a Gq-coupled receptor directly inhibits ion channel activity. Deletion of Ga_q largely abolished inhibition of TRPM8, and inhibition was rescued by a Ga_q chimera whose ability to activate downstream signalling pathways was completely ablated. Activated Ga_q protein, but not Gβγ, potently inhibits TRPM8 in excised patches. We conclude that Ga_q pre-forms a complex with TRPM8 and inhibits activation of TRPM8, following activation of G-protein coupled receptors, by a direct action. This signalling mechanism may underlie the abnormal cold sensation caused by inflammation.

Function and expression pattern of TRPM8 in bladder afferent neurons associated with bladder outlet obstruction in rats

We investigated the function and expression pattern of the transient receptor potential melastatin-8 (TRPM8) in urinary bladder afferent neurons from control and bladder outlet obstruction (BOO) rats. BOO was produced and, after six weeks, the effects of intravesical infusion of menthol, the agonist of TRPM8, were investigated using unanesthetized cystometry. The intravesical infusion of menthol produced an increase in the micturition pressure in both sham surgery and BOO rats. In BOO rats, increased basal and threshold pressure and a decreased micturition interval were observed. Next, the population of TRPM8-positive and the co-expression proportion of TRPM8 with neurochemical markers (NF200 or TRPV1) in the bladder afferent neurons were each compared between the control and BOO rats using retrograde tracing and immunohistochemistry. The population of TRPM8-immunoreactive bladder afferent neurons was larger in BOO rats (3.28±0.43%) than in the control rats (1.33±0.18%). However, there were no statistical differences between the control and BOO rats in the co-expression proportion of neither TRPM8-NF200 (84.1±4.3% vs 79.7±2.7%, p=0.41) nor TRPM8-TRPV1 (33.3±3.6% vs 40.8±2.6%, p=0.08) in the bladder afferent neurons. The present results suggest that the neuronal input through TRPM8-positive bladder afferent neurons are augmented after BOO, however, the neurochemical phenotype of the up-regulated TRPM8-positive bladder afferent neurons is not changed after BOO.

The design and synthesis of novel, phosphonate-containing transient receptor potential melastatin 8 (TRPM8) antagonists

A series of benzothiophene-based phosphonates was synthesized and many analogs within the series were shown to be potent antagonists of the TRPM8 channel. The compounds were obtained as a racemic mixture in 5 synthetic steps, and were tested for TRPM8 antagonist activity in a recombinant, canine TRPM8-expressing cell line using a fluorometric imaging plate reader (FLIPR) assay. Structure-activity relationships were developed initially by modification of the core structure and subsequently by variation of the aromatic substituents and the phosphonate ester. Compound 9l was administered intraperitoneally to rats and demonstrated engagement of the TRPM8 target in both prevention and reversal-modes in an icilin-induced 'wet-dog' shake model.

Transcranial magnetic stimulation reduces nociceptive threshold in rats

Transcranial magnetic stimulation (TMS) is a procedure that uses magnetic fields to stimulate or inhibit nerve cells in the brain noninvasively. TMS induces an electromagnetic current in the underlying cortical neurons. Varying frequencies and intensities of TMS increase or decrease excitability in the cortical area directly targeted. It has been suggested that TMS has potential in the treatment of some neurological disorders such as Parkinson's disease, stroke, and depression. Initial case reports and open label trials reported by several groups support the use of TMS in pain treatment. In the present study, we evaluated the effect of TMS on the nociceptive threshold in the rat. The parameters used were a frequency of 60 Hz and an intensity of 2 and 6 mT for 2 hr twice per day. After 5 days of TMS treatment, rats were evaluated for mechanical, chemical, and cold stimulation. We observed a significant reduction in the nociceptive threshold in TMS-treated rats but not in sham-treated rats in all behavioral tests evaluated. When TMS treatment was stopped, a slow recovery to normal mechanic threshold was observed. Interestingly, i.c.v. MK-801 or CNQX administration reverted the TMS-induced pronociception. The results suggest that high-frequency TMS can alter the nociceptive threshold and produce allodynia in the rats; results suggest the involvement of NMDA and AMPA/KA receptors on TMS-induced allodynia in the rat.

Role of TRPM8 in dorsal root ganglion in nerve injury-induced chronic pain

Background

Chronic neuropathic pain is an intractable pain with few effective treatments. Moderate cold stimulation can relieve pain, and this may be a novel train of thought for exploring new methods of analgesia. Transient receptor potential melastatin 8 (TRPM8) ion channel has been proposed to be an important molecular sensor for cold. Here we investigate the role of TRPM8 in the mechanism of chronic neuropathic pain using a rat model of chronic constriction injury (CCI) to the sciatic nerve.

Results

Mechanical allodynia, cold and thermal hyperalgesia of CCI rats began on the 4th day following surgery and maintained at the peak during the period from the 10th to 14th day after operation. The level of TRPM8 protein in L5 dorsal root ganglion (DRG) ipsilateral to nerve injury was significantly increased on the 4th day after CCI, and reached the peak on the 10th day, and remained elevated on the 14th day following CCI. This time course of the alteration of TRPM8 expression was consistent with that of CCI-induced hyperalgesic response of the operated hind paw. Besides, activation of cold receptor TRPM8 of CCI rats by intrathecal application of menthol resulted in the inhibition of mechanical allodynia and thermal hyperalgesia and the enhancement of cold hyperalgesia. In contrast, downregulation of TRPM8 protein in ipsilateral L5 DRG of CCI rats by intrathecal TRPM8 antisense oligonucleotide attenuated cold hyperalgesia, but it had no effect on CCI-induced mechanical allodynia and thermal hyperalgesia.

Conclusions

TRPM8 may play different roles in mechanical allodynia, cold and thermal hyperalgesia that develop after nerve injury, and it is a very promising research direction for the development of new therapies for chronic neuroapthic pain.

Sensing hot and cold with TRP channels

The past decade has witnessed the cloning of a new family of ion channels that are responsive to temperature. Six of these transient receptor potential (TRP) channels are proposed to be involved in thermosensation and are located in sensory nerves and skin. The TRPV1, TRPV2, TRPV3, and TRPV4 channels have incompletely overlapping functions over a broad thermal range from warm to hot. Deletion of the individual TRPV1, TRPV3, and TRPV4 channels in mice has established their physiological role in thermosensation. In all cases thermosensation is not completely abolished - suggesting some functional redundancy among the channels. Notably, the TRPV2 channel is responsive to hot temperatures in heterologous systems, but its physiological relevance in vivo has not been established. Cool and cold temperatures are sensed by TRPM8 and TRPA1 family members. Currently, the pharmaceutical industry is developing agonists and antagonists for the various TRP channels. For instance, TRPV1 receptor agonists produce hypothermia, while antagonists induce hyperthermia. Recent investigations have found that different regions of the TRPV1 receptor are responsive to temperature, nociceptive stimuli, and various chemical agents. With this information, it has been possible to develop a TRPV1 compound that blocks responses to capsaicin and acid while leaving temperature sensitivity intact. These channels have important implications for hyperthermia research and may help to identify previously unexplored mechanisms in different tissues that are responsive to thermal stress.

Scraping through the ice: uncovering the role of TRPM8 in cold transduction

The proper detection of environmental temperatures is essential for the optimal growth and survival of organisms of all shapes and phyla, yet only recently have the molecular mechanisms for temperature sensing been elucidated. The discovery of temperature-sensitive ion channels of the transient receptor potential (TRP) superfamily has been pivotal in explaining how temperatures are sensed in vivo, and here we will focus on the lone member of this cohort, TRPM8, which has been unequivocally shown to be cold sensitive. TRPM8 is expressed in somatosensory neurons that innervate peripheral tissues such as the skin and oral cavity, and recent genetic evidence has shown it to be the principal transducer of cool and cold stimuli. It is remarkable that this one channel, unlike other thermosensitive TRP channels, is associated with both innocuous and noxious temperature transduction, as well as cold hypersensitivity during injury and, paradoxically, cold-mediated analgesia. With ongoing research, the field is getting closer to answering a number of fundamental questions regarding this channel, including the cellular mechanisms of TRPM8 modulation, the molecular context of TRPM8 expression, as well as the full extent of the role of TRPM8 in cold signaling in vivo. These findings will further our understanding of basic thermotransduction and sensory coding, and may have important implications for treatments for acute and chronic pain.

TRPM8, but not TRPA1, is required for neural and behavioral responses to acute noxious cold temperatures and cold-mimetics in vivo

Somatosensory neurons detect environmental stimuli, converting external cues into neural activity that is relayed first to second-order neurons in the spinal cord. The detection of cold is proposed to be mediated by the ion channels TRPM8 and TRPA1. However, there is significant debate regarding the role of each channel in cold-evoked pain, complicating their potential as drug targets for conditions such as cold allodynia and hyperalgesia. To address this debate, we generated mice lacking functional copies of both channels and examined behaviors and neural activity in response to painful cold and noxious cooling compounds. Whereas normal mice display a robust preference for warmth over cold, both TRPM8-null (TRPM8(-/-)) and TRPM8/TRPA1 double-knockout mice (DKO) display no preference until temperatures reach the extreme noxious range. Additionally, in contrast to wildtype mice that avoid touching cold surfaces, mice lacking TRPM8 channels display no such avoidance and explore noxious cold surfaces, even at 5 degrees C. Furthermore, nocifensive behaviors to the cold-mimetic icilin are absent in TRPM8(-/-) and DKO mice, but are retained in TRPA1-nulls (TRPA1(-/-)). Finally, neural activity, measured by expression of the immediate-early gene c-fos, evoked by hindpaw stimulation with noxious cold, menthol, or icilin is reduced in TRPM8(-/-) and DKO mice, but not in TRPA1(-/-) animals. Thus our results show that noxious cold signaling is exclusive to TRPM8, mediating neural and behavioral responses to cold and cold-mimetics, and that TRPA1 is not required for acute cold pain in mammals.

The transient receptor potential channel TRPM8 is inhibited via the alpha 2A adrenoreceptor signaling pathway

The transient receptor potential channel melastatin member 8 (TRPM8) is expressed in sensory neurons, where it constitutes the main receptor of environmental innocuous cold (10-25 degrees C). Among several types of G protein-coupled receptors expressed in sensory neurons, G(i)-coupled alpha 2A-adrenoreceptor (alpha 2A-AR), is known to be involved in thermoregulation; however, the underlying molecular mechanisms remain poorly understood. Here we demonstrated that stimulation of alpha 2A-AR inhibited TRPM8 in sensory neurons from rat dorsal root ganglia (DRG). In addition, using specific pharmacological and molecular tools combined with patch-clamp current recordings, we found that in heterologously expressed HEK-293 (human embryonic kidney) cells, TRPM8 channel is inhibited by the G(i) protein/adenylate cyclase (AC)/cAMP/protein kinase A (PKA) signaling cascade. We further identified the TRPM8 S9 and T17 as two key PKA phosphorylation sites regulating TRPM8 channel activity. We therefore propose that inhibition of TRPM8 through the alpha 2A-AR signaling cascade could constitute a new mechanism of modulation of thermosensation in both physiological and pathological conditions.

Structural determinants of gating in the TRPV1 channel

Transient receptor potential vanilloid 1 (TRPV1) channels mediate several types of physiological responses. Despite the importance of these channels in pain detection and inflammation, little is known about how their structural components convert different types of stimuli into channel activity. To localize the activation gate of these channels, we inserted cysteines along the S6 segment of mutant TRPV1 channels and assessed their accessibility to thiol-modifying agents. We show that access to the pore of TRPV1 is gated by S6 in response to both capsaicin binding and increases in temperature, that the pore-forming S6 segments are helical structures and that two constrictions are present in the pore: one that impedes the access of large molecules and the other that hampers the access of smaller ions and constitutes an activation gate of these channels.

Transient receptor potential channels: targeting pain at the source

Pain results from the complex processing of neural signals at different levels of the central nervous system, with each signal potentially offering multiple opportunities for pharmacological intervention. A logical strategy for developing novel analgesics is to target the beginning of the pain pathway, and aim potential treatments directly at the nociceptors--the high-threshold primary sensory neurons that detect noxious stimuli. The largest group of receptors that function as noxious stimuli detectors in nociceptors is the transient receptor potential (TRP) channel family. This Review highlights evidence supporting particular TRP channels as targets for analgesics, indicates the likely efficacy profiles of TRP-channel-acting drugs, and discusses the development pathways needed to test candidates as analgesics in humans.

Mechano- and thermosensitivity of regenerating cutaneous afferent nerve fibers

Crush lesion of a skin nerve is followed by sprouting of myelinated (A) and unmyelinated (C) afferent fibers into the distal nerve stump. Here, we investigate quantitatively both ongoing activity and activity evoked by mechanical or thermal stimulation of the nerve in 43 A- and 135 C-fibers after crush lesion of the sural nerve using neurophysiological recordings in anesthetized rats. The discharge patterns in the injured afferent nerve fibers and in intact (control) afferent nerve fibers were compared. (1) Almost all (98%) A-fibers were mechanosensitive, some of them exhibited additionally weak cold/heat sensitivity; 7% had ongoing activity. (2) Three patterns of physiologically evoked activity were present in the lesioned C-fibers: (a) C-fibers with type 1 cold sensitivity (low cold threshold, inhibition on heating, high level of ongoing and cold-evoked activity; 23%): almost all of them were mechanoinsensitive and 40% of them were additionally heat-sensitive; (b) C-fibers with type 2 cold sensitivity (high cold threshold, low level of ongoing and cold-evoked activity; 23%). All of them were excited by mechanical and/or heat stimuli; (c) cold-insensitive C-fibers (54%), which were heat- and/or mechanosensitive. (3) The proportions of C-fibers exhibiting these three patterns of discharge to physiological stimuli were almost identical in the population of injured C-fibers and in a population of 91 intact cutaneous C-fibers. 4. Ongoing activity was present in 56% of the lesioned C-fibers. Incidence and rate of ongoing activity were the same in the populations of lesioned and intact type 1 cold-sensitive C-fibers. The incidence (but not rate) of ongoing activity was significantly higher in lesioned type 2 cold-sensitive and cold insensitive C-fibers than in the corresponding populations of intact C-fibers (42/93 fibers vs. 11/72 fibers).

Cutaneous afferent C-fibers regenerating along the distal nerve stump after crush lesion show two types of cold sensitivity

Cutaneous C-fiber afferents show two distinct types of cold sensitivity corresponding to non-noxious and noxious cold sensations. Here, responses to cold stimulation of afferent fibers regenerating in the rat sural nerve were studied in vivo 7-14 days after nerve crush and compared with responses to mechanical and heat stimulation. The physiological stimuli were applied to the sural nerve at or distal to the lesion site. Ectopic activity was evoked in 43% of 98 A-fibers (all mechanosensitive; a few additionally weakly thermosensitive). Ectopic activity was evoked in 127 (49.2%) of 258 electrically identified C-fibers by the physiological stimuli. Eight C-fibers were spontaneously active only. Of the 127 C-fibers, 46% had one of two distinct response patterns to cooling: (1) type 1 cold-sensitive C-fibers (n=29) had a high rate of activity at 28 degrees C on the nerve surface and showed graded responses to cooling with maximal discharge rates of 11.5+/-1.1 imp/s. This activity was completely inhibited by heating, while 12/29 fibers were also excited at high threshold (median 48 degrees C) by heating. Only one type 1 cold-sensitive C-fiber was mechanosensitive. (2) Type 2 cold-sensitive C-fibers (n=29) were silent or showed a low rate of activity at 28 degrees C, had a high threshold (median 5 degrees C) and low maximal discharge rates (2.4+/-0.4 imp/s) to cooling. They were also heat-sensitive (n=25) and/or mechanosensitive (n=20). These C-fibers were, apart from their cold sensitivity, functionally indistinguishable from C-fibers with mechano- and/or heat sensitivity only. Thus regenerating cutaneous C-fibers show two types of cold sensitivity similar to those observed in intact skin: fibers of one group are predominantly sensitive to cooling, whereas the others are polymodal.

Visualizing cold spots: TRPM8-expressing sensory neurons and their projections

Environmental stimuli such as temperature and pressure are sensed by dorsal root ganglion (DRG) neurons. DRG neurons are heterogeneous, but molecular markers that identify unique functional subpopulations are mainly lacking. ThermoTRPs are members of the transient receptor potential family of ion channels and are gated by shifts in temperature. TRPM8 is activated by cooling, and TRPM8-deficient mice have severe deficits in cool thermosensation. The anatomical and functional properties of TRPM8-expressing fibers have not been not comprehensively investigated. We use mice engineered to express the farnesylated enhanced green fluorescent protein (EGFPf) from the TRPM8 locus (TRPM8(EGFPf)) to explore this issue. Virtually all EGFPf-positive cultured DRG neurons from hemizygous mice (TRPM8(EGFPf/+)) responded to cold and menthol. In contrast, EGFPf-positive DRGs from homozygous mice (TRPM8(EGFPf/EGFPf)) had drastically reduced cold responses and no menthol responses. In vivo, EGFPf-positive neurons marked a unique population of DRG neurons, a majority of which do not coexpress nociceptive markers. The fraction of DRG neurons expressing EGFPf was not altered under an inflammatory condition, although an increase in TRPV1-coexpressing neurons was observed. TRPM8(EGFPf) neurons project to the superficial layer I of the spinal cord, making distinct contacts when compared with peptidergic projections. At the periphery, TRPM8(EGFPf) projections mark unique endings in the most superficial layers of epidermis, including bush/cluster endings of the mystacial pads. We show that TRPM8 expression functionally associates with cold sensitivity in cultured DRGs, and provide the first glimpses of the unique anatomical architecture of cold fibers in vivo.

TRPM8 mechanism of cold allodynia after chronic nerve injury

The cold- and menthol-sensitive receptor TRPM8 (transient receptor potential melastatin 8) has been suggested to play a role in cold allodynia, an intractable pain seen clinically. We studied how TRPM8 is involved in cold allodynia using rats with chronic constrictive nerve injury (CCI), a neuropathic pain model manifesting cold allodynia in hindlimbs. We found that cold allodynic response in the CCI animals was significantly attenuated by capsazepine, a blocker for both TRPM8 and TRPV1 (transient receptor potential vanilloid 1) receptors, but not by the selective TRPV1 antagonist I-RTX (5-iodoresiniferatoxin). In L5 dorsal root ganglion (DRG) sections of the CCI rats, immunostaining showed an increase in the percentage of TRPM8-immunoreactive neurons when compared with the sham group. Using the Ca2+-imaging technique and neurons acutely dissociated from the L5 DRGs, we found that CCI resulted in a significant increase in the percentage of menthol- and cold-sensitive neurons and also a substantial enhancement in the responsiveness of these neurons to both menthol and innocuous cold. These changes occurred in capsaicin-sensitive neurons, a subpopulation of nociceptive-like neurons. Using patch-clamp recordings, we found that membrane currents evoked by both menthol and innocuous cold were significantly enhanced in the CCI group compared with the sham group. By retrograde labeling afferent neurons that target hindlimb skin, we showed that the skin neurons expressed TRPM8 receptors, that the percentage of menthol-sensitive/cold-sensitive/capsaicin-sensitive neurons increased, and that the menthol- and cold-evoked responses were significantly enhanced in capsaicin-sensitive neurons after CCI. Together, the gain of TRPM8-mediated cold sensitivity on nociceptive afferent neurons provides a mechanism of cold allodynia.

Diversity in the neural circuitry of cold sensing revealed by genetic axonal labeling of transient receptor potential melastatin 8 neurons

Sensory nerves detect an extensive array of somatosensory stimuli, including environmental temperatures. Despite activating only a small cohort of sensory neurons, cold temperatures generate a variety of distinct sensations that range from pleasantly cool to painfully aching, prickling, and burning. Psychophysical and functional data show that cold responses are mediated by both C- and A delta-fibers with separate peripheral receptive zones, each of which likely provides one or more of these distinct cold sensations. With this diversity in the neural basis for cold, it is remarkable that the majority of cold responses in vivo are dependent on the cold and menthol receptor transient receptor potential melastatin 8 (TRPM8). TRPM8-null mice are deficient in temperature discrimination, detection of noxious cold temperatures, injury-evoked hypersensitivity to cold, and nocifensive responses to cooling compounds. To determine how TRPM8 plays such a critical yet diverse role in cold signaling, we generated mice expressing a genetically encoded axonal tracer in TRPM8 neurons. Based on tracer expression, we show that TRPM8 neurons bear the neurochemical hallmarks of both C- and A delta-fibers, and presumptive nociceptors and non-nociceptors. More strikingly, TRPM8 axons diffusely innervate the skin and oral cavity, terminating in peripheral zones that contain nerve endings mediating distinct perceptions of innocuous cool, noxious cold, and first- and second-cold pain. These results further demonstrate that the peripheral neural circuitry of cold sensing is cellularly and anatomically complex, yet suggests that cold fibers, caused by the diverse neuronal context of TRPM8 expression, use a single molecular sensor to convey a wide range of cold sensations.

Icilin induces a hyperthermia in rats that is dependent on nitric oxide production and NMDA receptor activation

Icilin (AG-3-5) is a cold-inducing agent that activates the transient receptor potential channels TRPM8 and TRPA1. Both channels are members of the transient receptor potential (TRP) superfamily of ion channels and are activated by cold. Despite the key role of cold-activated TRPM8 and TRPA1 channels in temperature sensation and other physiological processes, the significance of these channels in thermoregulation in conscious animals is poorly understood. Therefore, in the present study we investigated the effects of icilin on body temperature in rats and tested the hypothesis that cold-activated TRP channel activation by icilin causes a hyperthermia which requires nitric oxide (NO) production and NMDA receptor stimulation. Our experiments revealed that icilin (2.5, 5, 7.5 and 10 mg/kg, i.m.) elicits a dose-related hyperthermia that is rapid in onset and of long duration. Pretreating rats with N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) (10, 25 and 50 mg/kg, i.p.), a non-selective NO synthase inhibitor, attenuated the hyperthermia associated with icilin (7.5 mg/kg, i.m.). Pretreatment with (-)-6-[phosphonomethyl-1,2,3,4,4a,5,6,7,8,8a-decahydro-isoquinoline-2-carboxylate] (LY 235959) (0.25, 0.5 and 1 mg/kg, i.p.), a selective NMDA receptor antagonist, also attenuated the icilin-evoked hyperthermia. The administration of icilin (5 and 100 microg) into the lateral cerebroventricle of rats did not affect body temperature, thus indicating a peripheral site of action. These results indicate that icilin, a TRPM8/TRPA1 agonist, produces a dose-related hyperthermia in rats which requires both NO production and NMDA receptor activation.

Mice left out in the cold: commentary on the phenotype of TRPM8-nulls

Detection of innocuous temperatures allows an organism to select an appropriate environmental climate, while the ability to recognize noxious temperature extremes warns of impending tissue damage. For temperatures considered cold, the menthol receptor TRPM8 is activated when temperatures drop below ~26°C, thus making it an intriguing candidate as the molecular mediator of cold perception. However, confirmation of this hypothesis in vivo has eluded researchers until recently. Three independent research groups have reported that mice lacking this single gene are severely impaired in their ability to detect cold temperatures. Remarkably, these animals are deficient in many diverse aspects of cold signaling, including cool and noxious cold perception, injury-evoked sensitization to cold, and cooling-induced analgesia. These animals provide a great deal of insight into the molecular signaling pathways that participate in the detection of cold and painful stimuli.